Valencia Lake, in northern Venezuela, and its tributary area (the catchment is approximately seven times larger than the lake which has an area of about 350 km2) are completely isolated from any river system or sea. The lake is about 400 m above sea level, and the adjacent ranges are up to 2,000 m higher. Observations of the fluctuations of the lake level since 1901 exist (with sporadic data since 1700), along with data on rainfall in the watershed and near the lake for the same period of time. Since 1939, 21 mostly small, tributary rivers have been hydrologically controlled. Evaporation (class A pan) has been observed also during this time. Since the tributary Rivers dry out for severall months each year, the lake descends as a function of its evaporation and serves as a large evaporation pan. The mean annual value is about 2,000 mm. By balancing losses (evaporation) against gains (runoff plus direct rain on the lake) a “hydrological pattern” was developed for the 61 years of fluctuation record; it was rechecked the last 22 years using the additional data. Yearly percentages as a function of the average were calculated.